1. Field of the Invention
The present invention relates to an ignition coil apparatus for an internal combustion engine that includes an ionic current detection unit for detecting, as an ionic current, the ions generated by the combustion of an air fuel mixture in each cylinder of the internal combustion engine,
2. Description of the Related Art
In recent years, there has been proposed a simultaneous ignition system that is designed to ignite a plurality of spark plugs at the same time by applying a high voltage for ignition to the plurality of spark plugs by means of a single ignition coil in order to achieve the reduction in the arrangement space of the ignition coil as well as the reduction in the production cost.
As one example of such a simultaneous ignition system, there is enumerated a combustion state detection apparatus for an internal combustion engine in which a high voltage for ignition is applied by means of one ignition coil to two spark plugs respectively arranged for two cylinders different from each other (see, for example, a first patent document: Japanese patent application laid-open No. 2000-205034).
The above-mentioned conventional apparatus is provided with the ignition coil (coil member), the spark plugs, a bias unit, a discharge current limiting unit, an ionic current detection unit, and an ECU (electronic control unit).
The ignition coil has a primary coil or winding and a secondary coil or winding, and generates a high voltage for ignition. The high ignition voltage thus generated is applied to the spark plugs. The bias unit is charged with a bias voltage of a positive polarity for detecting the ions generated by the combustion of an air fuel mixture in each of the engine cylinders. The discharge current limiting unit discharges the bias voltage thus charged to the bias unit. The ionic current detection unit detects the ions generated by the combustion of the air fuel mixture as an ionic current that flows through the spark plugs. The ECU detects a combustion state in each of the spark plugs based on the detected value of the ionic current.
Here, note that the discharge current limiting unit is arranged between an ignition current path, which extends from the secondary coil of the ignition coil to the spark plugs, and the bias unit.
In the above-mentioned conventional apparatus, the bias unit is charged by a voltage generated in the primary coil of the ignition coil at the instant when a primary current supplied to the primary coil is cut or interrupted. In addition, at this time, a high voltage for ignition is generated in the secondary coil. When combustion is generated by the high ignition voltage applied to the spark plugs in the cylinders immediately after the bias unit has been charged, the ionic current detection unit detects the ions generated in the cylinders as an ionic current.
Also, in recent years, there has been proposed a multi-point ignition system in which a plurality of spark plugs are installed for each of cylinders of an internal combustion engine and are ignited at a multiplicity of points within each cylinder in order to improve the combustion efficiency of the internal combustion engine.
Accordingly, it is considered that an ignition apparatus is constructed to have a multi-point ignition system by installing, on one cylinder, two spark plugs which are mounted on two mutually different cylinders, respectively, in the above-mentioned conventional apparatus.
In such an ignition apparatus, the ionic current detection unit can detect an ionic current through either of the spark plugs that are connected to a positive polarity side and a negative polarity side, respectively, of a secondary coil of the ignition coil.
In this ignition apparatus, when a primary current is started to be supplied to a primary coil of the ignition coil, there is generated in the secondary coil a voltage of a polarity opposite to the polarity of a voltage which is generated upon interruption of the primary current. That is, a voltage of a negative polarity (i.e., opposite to the direction or polarity of a regular bias voltage) is applied to the bias unit, and the bias voltage charged to the bias unit is discharged through the discharge current limiting unit.
In the above-mentioned ignition apparatus, at the instant when the primary current is started to be supplied to the primary coil, the negative polarity voltage is applied to the bias unit, whereby the bias voltage is discharged from the bias unit.
Therefore, there has been a problem that until the time when the bias unit has been recharged after the primary current is again interrupted, an ionic current can not be detected, and hence a combustion state in each of the spark plugs can not be detected, either.